In the catalytic cracking process, hydrocarbon feedstocks are converted under catalytic cracking conditions wherein at least a portion of the feedstock is converted to desired lighter hydrocarbon products. In general, catalytic cracking of the feedstock is undertaken in a fluidized bed, moving bed or fixed bed of catalyst at elevated temperatures generally ranging from about 850.degree. to 1300.degree.F. In the course of transforming the feedstock to more desirable products such as gasoline, liquefied petroleum gas, alkylation feedstocks and middle distillate blending stocks, a concomitant by-product formation occurs of an undesirable nature wherein coke is deposited on the catalyst. The substantial deposition of coke on a cracking catalyst reduces the catalyst's cracking activity and selectivity leading to a reduction is desired product formation.
The feedstocks employed in the catalytic cracking process consist of hydrocarbons that are liquids or solids at room temperature and which are at the catalytic cracking conditions in a fluid state, that is liquid or vapor. The desired products of the conversion are generally lower boiling materials. The feedstocks employed in the process can be derived from a plurality of sources such as petroleum, coal, shale oil or tar sands, including particular fractions of each of the above. The various sources of feedstock mentioned above and hydrocarbon fractions derived therefrom, particularly the heavier and higher boiling fractions, can contain in addition to the hydrocarbons metal contaminants which adversely affect the catalytic cracking process. A particularly deleterious contaminant found in the heavier fractions is vanadium, generally present as an organometallic compound, which in the course of the cracking reaction is deposited upon the catalyst in a form that substantially alters the selectivity and activity of the conversion process. The vanadium contaminant deposited on the catalyst unlike coke is not removed during regeneration. Continued use of the catalyst, therefore, causes progressively increasing quantities of vanadium to be deposited on the catalyst and such accumulation severely alters the selectivity and activity of the catalytic reactions. Another deleterious contaminant commonly found in the heavier fractions is nickel which is also known for its undesirable effect on the catalyst and cracking process. It will be appreciated that even low levels of metal contaminants in the feed cause the catalyst upon extended use to collect increasing quantities of the undesirable materials.
To overcome the debilitating effect of these metals on the catalyst and process, the art has proposed methods for demetalizing the catalyst employed in the catalytic cracking of contaminated feedstocks. Illustratively U.S. Pat. No. 3,151,088 discloses a method for removing vanadium, and also nickel, by vapor phase methods from conventional solid oxide cracking catalysts composed of, for example, synthetic gel silica-alumina. Essentially, the method for removing the contaminating metal involved sulfiding the catalyst and contacting the catalyst with carbon monoxide to remove nickel contaminant therefrom as volatile nickel carbonyl. Vanadium was also removed by chlorination at an elevated temperature employing a chlorinating reagent including molecular chlorine, chlorine substituted light hydrocarbons, sulfur chlorides, combinations of hydrogen sulfide and molecular chlorine, and hydrogen chloride and molecular chlorine. Prior to the chlorinating treatment, the coked catalyst was regenerated until the carbon content was less than about 5 weight percent and preferably less than about 0.5 weight percent.
The method described above for removing contaminants from the subject cracking catalyst, however, is unsuitable with regard to present day processes employing as cracking catalysts composites containing crystalline aluminosilicates or as they are sometimes referred to, zeolites. Employing the methods of the prior art when attempting to remove contaminants from present day catalysts, will contribute to the destruction of the crystalline aluminosilicate structure of the catalyst and reduce catalyst activity and selectivity. The catalytic cracking process is also affected through a loss of conversion and selectivity to desired products.
It is therefore an object of this invention to provide a process for catalytically cracking hydrocarbon feedstocks containing metal contaminants.
Another object of this invention is to provide a method for removing metal contaminants from crystalline aluminosilicate cracking catalysts which have become contaminated by use in cracking feedstocks containing metal contaminants.
Yet another object of this invention is to provide a process for converting hydrocarbon feedstocks containing metal contaminants in the presence of crystalline aluminosilicate cracking catalysts.
Other objects and advantages will become apparent from a reading of the following detailed description.